Solid inkjet imaging systems generally use an electronic form of an image to distribute ink melted from a solid ink stick or pellet in a manner that reproduces the electronic image. In some solid inkjet imaging systems, the electronic image may be used to control the ejection of ink directly onto a media sheet. In other solid inkjet imaging systems, the electronic image is used to operate printheads to eject ink onto an intermediate imaging member. A media sheet is then brought into contact with the intermediate imaging member in a nip formed between the intermediate member and a transfer roller. The heat and pressure in the nip help transfer the ink image from the intermediate imaging member to the media sheet, which is transported from the system and deposited in a paper tray.
In solid ink imaging systems having intermediate imaging members, ink is loaded into the system in a solid form, either as pellets or as ink sticks, and transported through a feed chute by a feed mechanism for delivery to a melting device. The melting device heats the solid ink to its melting temperature and the liquid ink is delivered to a printhead for jetting onto an intermediate imaging member. In the print head, the liquid ink is typically maintained at a temperature that enables the ink to be ejected by the printing elements in the print head, but that preserves sufficient tackiness for the ink to adhere to the intermediate imaging member. In some cases, however, the tackiness of the liquid ink may cause a portion of the ink to remain on the intermediate imaging member after the image is transferred onto the media sheet and the residual ink may later degrade other ink images formed on the intermediate imaging member.
In continuous-web, direct-to-media printing, a fixing assembly is used after the ink is ejected onto the print media or web to fix the ink to the web. The fixing assembly used depends on the type of ink. For example, when using melted phase change ink to form images, the fixing assembly may include a pair of rollers or drums that define a nip for applying pressure to the ink and web to spread the ink on the web as the web passes through the nip, as depicted in FIG. 1. The function of the pair of rollers, also referred to herein as a spreader, is to transform a pattern of ink drops deposited onto a web by flattening and spreading the ink drops to make a more uniform and continuous layer. The spreader uses pressure and heat to reduce the height of the ink droplets and fill the spaces between adjacent drops.
One difficulty faced in the operation of the spreader is providing the web and the ink deposited on the web to the spreader at a temperature that enables the ink deposited on the web to be spread uniformly for high image quality. Due to very fast processing speeds at which some continuous feed imaging devices operate, the ink deposited on the web at the print station may be above a suitable temperature range as the image passes through the nip. This high ink temperature results in the ink bleeding into the web and possibly showing through to the opposite side of the media web. Conversely, if the ink cools below the suitable temperature range prior to reaching the spreader, the ink may not be malleable enough to allow for sufficient line spread or adherence to the web. In addition, the ink ejected by the printheads is generally much hotter than the print medium, and, consequently, areas imaged with high ink coverage may exit from the print zone at higher temperatures than the areas of the media web where little or no ink was ejected. Ink that enters the spreader at varying temperatures can cause inconsistent and non-uniform line spread on the web, reducing image quality. Thus, improved media and ink temperature equalization is desirable. For optimum spreader performance, ink and web temperatures are substantially equalized prior to entering the nip to within a target temperature range that promotes adherence of the melted ink to the web, minimizes visibility of printed ink from the opposite side of the media (“show-through”), maximizes ink dot spread, and reduces image defects on the opposite side of the media in a duplex printing process. The target temperature range for the ink and web prior to entering the nip can also be referred to as the pre-spreading temperature range. In one embodiment, the pre-spreading temperature range is between about 50° C. and about 55° C. The pre-spreading temperature range, however, can be any suitable range of temperatures suitable for spreading ink on a web depending on factors such as the ink formulation, web substrate material, web velocity, and the like.
To address and/or prevent the accumulation of ink on an intermediate imaging member or on a spreader, which may be in the form of a drum, solid ink imaging systems may be provided with a drum maintenance unit (DMU), as shown in FIG. 1, that releases an oil or oil blend that lubricates the image receiving surface of the intermediate imaging member before each print cycle, thereby preventing ink offset to the spreader drum. Typically, these DMU oils are silicone oils that may contain additives to allow for proper lubrication and release characteristics.
Some print machines are capable of duplex printing, namely printing on both sides of the print media or web. Other print machines utilize separate similarly configured print engines, such as the upstream and downstream in-line print engines depicted in the diagram of FIG. 2. Tandem duplex print lines such as the system depicted in FIG. 2 provide high speed duplex printing using a fixing assembly such as the fixing assembly depicted in FIG. 1. Consequently, the upstream and downstream print engines each have their own DMU for transferring the release oil formulation to the spreader drum. As mentioned, oiling the spreader drum is necessary to prevent ink offset to the spreader drum and to other duplex sensitive components during duplex printing.
However, in some printing processes, the print media is subject to further processing downstream of the printing machine. For instance, a “post-printing” coating, such as a varnish, may be applied to the printed media or web to enable acceptable print durability. In some processes, the post-printing coating is necessary to avoid damage to the image in subsequent finishing or converting equipment, such as might be used in a direct-mail process for instance. The typical DMU formulation tends to remain on the surface of the print media, rather than soaking in, which interferes with the ability of the coating solution (e.g., varnish) to properly “wet” the surface of the printed material, particularly when the coating solution is applied during a high-speed in-line process. This interference thus results in poor coating uniformity and effectiveness of the “post-printing” coating.
As such, a DMU oil formulation for use in solid ink-jet color web-press printers that provides sufficient lubrication to an image receiving surface of the intermediate imaging member and image fixing members (i.e. a spreader drum) without interfering with downstream production finishing or converting equipment would be appreciated in the art.